Electrical photography



March 19, 1963 c. s. MILLER EAL 3,082,085

ELECTRICAL PHOTOGRAPHY Filed April 27, 1959 Canducf/re (arr/Fen CARL S. MILLER BYRON W. NEHER 4rrom/irs United States Patent 3,082,085 ELECTRICAL PHOTOGRAPHY Carl S. Miller, St. Paul, Minn., and Byron W. Neher, Hudson, Wis., assignors to Minnesota Mining and Manufacturing Company, St. Paul, Minn, a corporation of Delaware Filed Apr. 27, 1959, Ser. No. 809,134 5 Claims. (Cl. 96-1) This invention relates to a graphic reproduction system. In one aspect this invention relates to a process involving the directing of a light-image of an original onto a photosensitive surface and permanently altering the lightstruck areas of said surface to provide the desired reproduction.

The process is applicable to direct photography as well as to the reproducing of printed matter, sketches, paintings or other documents, and is operable with infrared, actinic light, ultraviolet light, X-rays, and other means of radiation. Unlike silver halide photography, the process of the present invention is capable of producing a completed permanent graphic reproduction simultaneously with exposure to the corresponding light-image without the subsequent application of chemical developing and fixing solutions. While appearing in some respects much more comparable to certain electrostatic copy-processes than to those concerned with silver halide reactions, the present invention avoids the use of developing powders or dusts and instead relies on physical or chemical means -to provide the desired reproduction on a photosensitive surface.

The object of this invention is to provide a new and useful reproduction system. 7

Yet another object is to provide a new reproduction composition.

Another object of this invention is to provide a dry process for the reproduction of images or copying of printed matter and the like.

Another object of this invention is to provide a process which directly reproduces the image or directly copies upon exposure to the object to be reproduced and does not necessarily require a separate step of developing.

Yet another object of this invention is to provide a process which utilizes actinic light to reproduce a permanent image on a suitable carrier without a separate fixing step to make the image permanent.

Another object is to provide a new copy or reproduction paper.

Various other objects and advantages will become apparent to those skilled in the art from the accompanying description and disclosure.

According to this invention a carrier or support containing an irradiation-sensitive composition is exposed to an irradiation source, such as through a negative, to produce an image or pattern in latent form. Simultaneously or thereafter, the latent image on the exposed reproduction carrier is subjected to a high potential gas iion discharge which develops the image visibly. The ion discharge consists essentially of normally gaseous ions which are negatively charged. The source of the gas ion discharge is a corona which is connected to a negative source of direct current. The carrier of the reproduction composition is grounded or connected to a positive source of direct current.

The bombardment of the surface of the reproduction carrier with the negatively charged gas ions from the corona source reproduces the image as the result of a differential conductive pattern in the areas of the irradiation-sensitive materials which have been exposed to the image source and which as a result show either more or less resistivity to the conduction of the ions to the ground or to the positive terminal. The gas ions from the corona 3,082,085 Patented Mar. 19, 1963 "ice discharge hit the surface of the photosensitive material at those points of highest conductivity (least resistivity) and cause a chemical reaction or a decomposition of the image forming composition resulting in a change in color or light value. No aerosol material or other matter, such as dyes, are necessary in the environmental atmosphere. The sole material which bombards the reproduction carrier is normally gaseous ions, such as oxygen of the air.

A critical aspect of this invention is the use of a negatively charged electrode. Image formation will not result if the electrode or corona is positively charged or grounded and the reproduction carrier negatively charged.

The carrier upon which the image is reproduced contains an electrically conductive base, such as metal, upon which the irradiation-sensitive material, such as a photoconductor, is placed or bonded thereto. Other chemicals as hereinafter defined may be included as a part of the composition on the conductive carrier. The irradiationsensitive material on the carrier is the only conductor present other than the electrically conductivebase. Any binders; or other chemicals introduced into the system must be nonconductors as at least compared with the conductivity of the irradiation-sensitive material in its most conductive state. It is important in a relative sense that the irradiation-sensitive material, such as a photoconductor, is the only material capable of conduction under the conditions of ion discharge on the surface of the conductive base. H

Exposure to the light-image and to the electrical discharge may take place simultaneously, or the electrical discharge may be applied subsequent to exposure to the light-image, The light-struck areas of a White or lightcolored copy-paper may be darkened, as in forming a positive print from a photographic negative transparency; or the light-struck areas of a dark-colored copy.- paper may be lightened in color, as in forming a direct positive reproduction of an opaque original.

Metal foil paper laminate is a suitable carrier or base material. Metal conductors, such as aluminum, chromium, nickel and copper, are suitable as the conductive material on the carrier. Other conductive sheet material may be substituted for the foil paper laminate. Aluminum foil or other metal foils alone are excellent conductors but are expensive and unpleasant to handle. Paper, plastic films, or other poorly conductive carriers or flexible sheets or films may be rendered adequately conductive by surface-coating with aluminum or other metal applied in vapor form under vacuum. In some cases, particularly under conditions of high humidity, many papers or films are adequately conductive without further treatment. One exampleis regenerated cellulose film (cellophane) which carries a content of glycerine or like moisture absorbing material. The use of a transparent plastic film upon which is deposited a thin transparent layer of metal conductor is advantageous, so that the corona discharge and the light-image is applied from opposite sides of the carrier or film to produce an'efiective copy or transparency while avoiding any possibility of shadows resulting from the electrode.

' Nonconductive sheets or films have been found use ful, where of suificient thinness, by pressing them into close uniform contact with a conductive base. The combination of the nonconductive thin plastic film and a metal foil base is useful as a carrier material for the sensitive coating.

The sensitive coating is also applicable to various ar ticles other than thin sheets or films, including metal plates, clock faces or instrument dials, etc.

Any combination of photoconductor and electrically conductive carrier maybe shown to be photoelectrosensitive by connecting the conductive paper--or the con ductive base or clamp to which it is attached-to the source of potential through a sensitive current-measuring device, and noting any change in current occurring with change in illumination of the copying-paper surface. The value of the current is observed to change abruptly when the previously unlighted photoelectrosensitive copyingpaper is illuminated. Most such sheets show an increase in current flow on illumination; others show a decrease. Both types are useful.

The irradiation-sensitive agents which may be used in accordance with this invention include the photoconductors, such as anthracene, anthraquinone, sulfur, zinc sulfide, yellow cadmium sulfide, zinc oxide, a mixture of zinc oxide, zinc sulfide and cadmium sulfide, a mixture of zinc and cadmium sulfide, titanium dioxide, barium titanate, selenium, bismuth trioxide, lead oxide and mercuric oxide. Mixtures of the above photoconductors may be effectively used without departing from the scope of this invention. All of these materials show a change in conductivity or resistivity upon exposure to irradiation. Certain of these materials; namely, bismuth trioxide, titanium dioxide, barium titanate and lead oxide, are themselves capable of providing a visible change on simultaneous ex posure to light and gas ion discharge without the use of a color-changing agent. Others, such as zinc oxide, are not visibly affected but a latent image may be formed, and sensitive coatings employing this and similar photo conductive materials must, therefore, be provided with color-changing or photodeveloper agents for imparting the desired permanent visible change. many effective materials have been found. These include nonconductive organic dyes, such as Victoria Green WB Crystals (a triphenylmethane dye, Color Index 657), Cal cozine Red BX (Color Index 749), Erie Red 43 (Color Index 448), Congo red (an azo dye), and Pontachrome Blue ECR (Color Index 722); and oxidation-reduction indicators such as diphenylamine, diphenylbenzidine, N,- N-diethyl p-phenylene diamine or its hydrochloride, diphenylguanidine, diphenyl urea, pand o-phenylene diamine, and p-phenetidine. Preferably, these dyes are oxidizable under the conditions of gas ion discharge. These materials either react or decompose to effect the color change.

The thickness of the image-reproducing composition on the surface of the carrier or conductive base will be between about 0.5 and about 8 mils. In case separate materials are used, such as a photoconductor and a dye, these materials may be admixed and placed upon the carrier as a mixture or they may be placed upon the carrier in layers. However, the layer adjacent or in contact with the electrically conductive base must be the photoconductive material. The thickness of the carrier, including the electrical conductor, when in paper form will be between about 2 and about 50 mils in thickness, generally less than 30 mils in thickness.

The exposure time used to reproduce the image will vary to a considerable extentand will depend primarily upon the type and intensity of the light or irradiation source, the sensitivity of the image-reproducing composition and/or upon the sensitivity of the photoconductor. In general, the time of exposure will vary between about 0.01 and about 30 minutes. The image-reproducing composition and the sheet material made therefrom should be conditioned in the dark before exposure and should not be exposed to the light prior to exposure for reproducing the image. The exposure of the image-reproducing composition to the gas ion discharge to develop the latent image will be between about 0.01 and about 120 minutes, usually less than 30 minutes and may be done simul: taneously with the initial exposure to the image.

'In most instances, no fixing or inactivation of the image-forming composition is necessary when viewed under ordinary actinic light conditions. However, this will depend upon the sensitivity of the composition to actinic light. If the image-forming composition is sensitive to For this purpose,

actinic light, removal or inactivation of the image-forming composition may be necessary, such as by washing off the unreacted materials. These compositions described herein are usually not sufficiently sensitive to ordinary light to cause darkening of the background or fading of the image.

The invention will be further described in connection with the accompanying drawing. FIGURE 1 diagrammatically illustrates typical apparatus employed in making a reproduction of a photographic transparency or the like FIGURE 2 indicates a cross section of a representative form of irradiation-sensitive copy-paper. The sensitive copysheet 10, comprising an outer photoconductive layer 11 and an electrically conductive backing or carrier 12, such as a paper-aluminum foil laminate, is supported against a suitable base 13 with the conductive carrier 12 connected to ground. An electrode 14 is supplied with high voltage (at least 1,000 volts, preferably at least 10,000 volts) from a source 15 which is also grounded, the system providing a high potential gradient and an ion discharge in the direction of the copysheet 10'. Actinic light or other type irradiation from a source 16 is directed by a concave mirror 17 through the transparent areas of a photographic negative or other suitable graphic original 18 to produce a light-image which is focused, by means of a stop 19 and a lens 20, on the irradiation-sensitive sur face 11 of the sheet 10.

In making a copy, for example of an original in the form of a photographic transparency such as a microfilm slide, the light-image produced in the lamp and lens system is focused on the sensitive surface 11 of sheet 10, which at the same time is subjected to the gas ion discharge from point electrode 14. A visible image is rapidly produced on the sensitive surface. Alternatively, with copy-paper of suitable proerties, the sheet is first exposed to the light-image with the electrode removed, and the electrode is subsequently replaced and voltage applied to develop the copy.

Typical apparatus which has been employed in procedures as just described includes a conventional photoprojector employing a 500 watt lamp and an M5 lens having a focal length of three inches, the projector being at a distance of about 20 inches from the flat base 13. A photographic transparency, e.g., a lantern slide, is used as the transparency 18. The electrode '14 is a sharpended small-diameter steel wire, the point being located centrally of the picture area and at a distance of about 1 to 4 inches, preferably about Z-inches, from the face of the copy-paper 10. The negative lead of a conventional DC. power supply at a potential of 15-25 kilovolts is connected to the electrode 14; the positive terminal is grounded or is connected directly to the conductive backing of the copy-paper 10, which is suitably supported on the flat base 13. The apparatus is maintained in darkness or in subdued light during the copying procedure.

The term light as applied to the radiation provided from the projector 10 will be understood -to include monochromatic light and various invisible radiations as well as the white light ordinarily employed for image projection. Ultraviolet light, in the neighborhood of 3600 Angstroms wavelength, has proven particularly suitable; and radiations of still higher frequency, such as X-rays and radiations from radioactive sources, are also useful under somewhat different but analogous conditions. In all cases, the radiation used must be capable of producing the required differential conductive effect at the receptor surface containing the compound which undergoes a change in light value.

The single corona point source 14 shown in FIGURE 1 may be replaced by a network of points or line sources of corona, and such network may be caused to move across the light-beam in order to avoid the formation of shadows on the sensitive sheet, to permit the printing of larger areas, to provide a more uniform corona, and to accelerate the speed of reproduction.

Particulate irradiation-sensitive materials, such as zinc oxide, are preferably applied as dispersions in small amounts of water-insoluble nonconductive film-forming binders, as shown in the examples. The binder holds the particles to the conductive backing and appears to aid in insulating the particles from each other and prevents the gas ions from dissipation over the entire surface. Small amounts of a nonconductive binder, just sufiicient to form a well-bonded, rub-resistant coating, provide maximum potential differential at the coated surface and are preferred. Useful copying-papers may be produced in which the dispersion is distributed within the fibrous sheet, or in which the irradiation-sensitive particles are incorporated in the sheet, in the substantial absence of binder, during the formation of the paper from the fibrous wood or cloth pulp. On the other hand, the binder layer may first be applied to the conductive film or sheet and the particulate irradiation-sensitive material adhered to the surface; or a surface film of sensitive material may be formed, e.g., by vapor or solvent deposition of selenium, or by converting surface films of zinc or cadmium to sulfide form by appropriate chemical treatment.

Other materials may be included with the photo-electrosensitive material, in these various constructions, for a variety of purposes. For example, nonconductive waterresistant soaps, such as zinc stearate or cobalt palmitate, are included to improve the smoothness or eliminate the graininess of the coated surface, to improve the adhesion of the coating to metal surfaces, or to minimize the effects of soluble salts or other materials present in the sheet, or to alter the sensitivity of the sheet to light of various wavelengths.

Specific illustrative but nonlimitative examples of sensitive copying-papers which have been found useful in the process hereinbefore described will now be given, together with further details of their application to the reproduction of light-images. Proportions appearing in these examples are given in parts by weight unless otherwise stated.

Example 1 A mixture of 55 parts by weight of photoconductive zinc oxide (Mercks reagent grade), 5 parts of a chlorinated rubber binder (Parlon, 125 cps, grade), and 40 parts of toluene (solvent) is ground in a pebble mill for 3-4 hours until a smooth dispersion is obtained. The dispersion is spread over the metal surface of a laminate of thin aluminum foil and paper, in a uniform layer and at a thickness of 3 mils (0.003 inch), and the solvent removed by evaporation at moderately elevated temperature. The coated surface is then swabbed with a 5% solution of diphenylamine in ethanol, and dried at room temperature. The thus sensitized sheet is white in appearance and remains substantially unchanged on exposure to normal light although some discoloration takes place on prolonged aging.

The sheet is supported on the flat base of a printing apparatus as described in connection with FIGURE 1 of the drawing and connection is made between the aluminum foil layer and the paper-holding clamps, which are A conductive laminate as employed in Example 1 is coated with a two-mil thickness of a smooth ballmilled blend of 48 parts by weight of zinc oxide, 4 parts of a resinous copolymer of butadiene and styrene (Pliolite 8-5) as a binder, and 48 parts of toluene as a solvent, and dried. The coated surface is swabbed with a 1% solution of Calcozine Green V dye in ethanol, and again dried. The sheet has a faint green appearance.

The sheet is exposed to a light-image formed through a photographic positive transparency while being subjected to corona discharge, as previously described. The sheet is converted to a permanent dark green color at the light-struck areas.

Example 3 A two-mill smooth coating of a uniform mixture of 50 parts powdered yellow bismuth trioxide (Bakers GP. grade), 5 parts chlorinated rubber, and 45 parts toluene is provided on a paper-aluminum foil laminate and dried to produce a pale yellow colored sensitive copying-paper. Exposure for about 5 minutes under the conditions previously described produces a permanent black image corresponding to the light-struck areas, on the yellow background. Some further darkening of the image occurs on continuing the exposure to 15 minutes or somewhat longer, and without observable darkening of background areas.

At higher lightintensities, the time requiredfor developing the permanent image is substantially reduced, images having been developed within an exposure time of 10 seconds without difficulty. Increased sensitivity, obtained by incorporating trace amounts of lead or cerium in the bismuth oxide pigment, likewise increases the speed of development. One such sensitized composition is 50 parts of Blgog, 5 parts of ceric stearate, 5 parts of chlorinated rubber (Parlon) and 40 parts of toluene (solvent).

Example 4 The sensitivity of the color change occurring in the sensitive copying-paper of Example 3 is increased by applying a thin coating of the bismuth trioxide'dispersion over a primary coating of zinc oxide as provided in Example 1.

Example 5 Barium titanate is substituted for the bismuth trioxide in the two-layer coated sheet of Example 4. The copying-paper, which is stable on exposure to bright daylight, is converted from an initial white or faint pink color to a black appearance on exposure to light and corona discharge under the conditions described. The image is rletziined for several days both in the light and in the Titanium dioxide is equally as eifective .as the barium titanate in the production of an image on exposure tc ion discharge in the presence of a light-image, and the resulting image is stable on prolonged storage.

, Example 6 The sensitizing coating in this example is formed of a uniform mixture of 50 parts yellow lead oxide, 5 parts chlorinated rubber, and parts-toluene, applied to a paper-metal foil laminate at a uniform thickness of .3 mils and dried. Exposure for 15 minutes produces 2 mildly black image of the light-struck areas on a yellovt background.

Example 7 A conductive backing coated with a thin dried layer of a dispersion of a zinc; cadmium sulfide phosphor powder (in this case New Jersey Zinc Cos. No. 2220) in 2 polymeric binder is surface-treated with a variety of dye: to form sensitive copying-papers useful in the practice 01 this invention. Typical dyes are Victoria Green WB Calcozine Green V and Calcozine Red BX.

While it is not desired to be limited to any theories of operation, it is believed that the following proposec' explanation'of the surprising results here obtained will aid in arriving at an understanding and appreciation o1 the principles underlying the invention. The light first activates the photoelectrosensitive particles, producing in said particles either a decrease or, more commonly, an increase in the conductivity of the particle. The high potential gradient establishes an electrostatic force across the photoeleetrosensitive layer. Any resulting charges at conductive areas of the surface are neutralized by conduction to or from the conductive backing or sup porting plate; but such action cannot occur, at least to the same extent, at nonconductive areas, and hence a differential potential pattern, corresponding to the nonlighted areas, is established on the exposed surface. Meanwhile, the corona set up at the high potential source causes ionization of the surrounding air, and particles or ions resulting from such ionization and carrying a suitable charge are drawn toward the oppositely charged areas of the reproduction paper. Chemical action or decomposition of the reactive components within or on the surface of the reproduction paper, resulting in the development of the image, is caused by the presence of reactive atoms, such as oxygen, or the high energy level at the surface of the reproduction, or by the presence of tree electrons.

Evidence in support of the above theory in the chemical action is oxidation and is found in the observed fact that the sheet material of Example 1 is inoperable in the photocopying method described when the air normally surrounding the high potential source is replaced by oxygen-free nitrogen. Diphenylamine is converted from light green to dark green by oxidation. It appears likely, therefore, that oxygen ions are responsible for the color change produced in Example 1; and such ions are known to be produced by corona discharge in air. Corona discharge may also produce ions of ozone and of oxides of nitrogen when produced in air, and other Ions when produced in other ionizable gases or mixtures of gases. Reactants which undergo a suitable visible change on reaction with such ions are contemplated, therefore, as photodeveloper components for the photoelectrosensitive coatings of the reproduction papers of this invention.

. In the foregoing specific examples the light-image is formed on the sensitive surface by focusing the light passing through a transparent original such as a lantern slide. In analogous manner, the light-image may be formed by reflection from an opaque printed surface or from an original article, by methods commonly employed in photography. The light may be filtered through larious colored filters for separating colored images into :nonochromatic components.

As has been previously stated, successful operation of :he process depends on' the use of a negatively charged :lectrode of suificiently high voltage to cause ion dis- :harge toward the copy-paper. It has, however, been ioted in one instance, i.e., with bismuth trioxide as the ehotoconductor, that similar reproduction results can be )btained with the electrode positievly charged.

Various combinations of irradiation-sensitive materials 1nd dyes may be employed Without departing from the 360136 of this invention. Various other modifications and tlterations will become obvious to those skilled in the art from the accompanying disclosure and description.

Having described our invention, we claim:

1. A method for reproduction of a graphic original comprising projecting the graphic original upon a photoconductive copy-sheet comprising a metal layer upon which has been bonded thereto a photoconductive layer containing a compound selected from at least one member of the group consisting of bismuth trioxide, titanium dioxide, barium titanate and lead oxide which undergoes a change in color upon reaction when subjected to a high potential negative ion bombardment created by a corona discharge of at least 10,000 volts to produce a differential conductive pattern comprising relatively non-conductive and relatively conductive areas on the surface thereof corresponding to said graphic original, establishing a high potential gradient of at least 10,000 volts between said copysheet containing said differential conductive pattern and a negative corona electrode in the presence of an ionizable atmosphere consisting essentially of normally gaseous material, said electrode being positioned at such a distance from said copy-sheet that substantially the entire conductive areas of said copy-sheet are bombarded with negatively charged normally gaseous ions, and continuing said bombardment of said copy-sheet with said negatively charged normally gaseous ions for sufficient length of time to produce a visible reproduction of said graphic original upon the surface of said copy-sheet.

2. The method of claim 1 in which the compound which undergoes a change in color is bismuth trioxide.

3. The method of claim 1 in which the compound which undergoes a change in color is titanium dioxide.

4. The method of claim 1 in which the compound which undergoes a change in color is barium titanate.

' 5. The method of claim 1 in which the compound which undergoes a change in color is lead oxide.

References Cited in the file of this patent UNITED STATES PATENTS 2,281,013 Talrney Apr. 26, 1942 2,408,475 Nickle Oct. 1, 1946 2,543,051 Oughton et al. Feb. 27, 1951 2,555,321 Dalton et al. June 5, 1951 2,638,422 Cooley et al May 12, 1953 2,692,178 Grandadam Oct. 19, 1954 2,692,948 Lion Oct. 26, 1954 2,707,162 Fries Apr. 26, 1955 2,809,225 Morehouse et al. Oct. 8, 1957 2,817,765 Hayford et al. Dec. 24, 1957 2,825,687 Preston et a1 Mar. 4, 1958 2,833,930 Walkup May 6, 1958 2,862,815 Sugarman Dec. 2, 1958 2,959,481 Kucera Nov. 8, 1960 2,963,335 Hall et al. Dec. 6, 1960 2,982,647 Carlson May 2, 1961 FOREIGN PATENTS 201,301 Australia 2.. Mar. 19, 1956 201,416 Australia Apr. 13, 1956 188,030 Great Britain Oct. 23, 1922 464,112 Great Britain Apr. 12, 1937 OTHER REFERENCES Mees: The Theory of the Photographic Process, Macmillan, 1954, pages 131-132.

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1. A METHOD FOR REPRODUCTION OF A GRAPHIC ORIGINAL COMPRISING PROJECTING THE GRAPHIC ORIGINAL UPON A PHOTOCONDUCTIVE COPY-SHEET COMPRISING A METAL LAYER UPON WHICH HAS BEEN BONDED THERETO A PHOTOCONDUCTIVE LAYER CONTAINING A COMPOUND SELECTED FROM AT LEAST ONE MEMBER OF THE GROUP CONSISTING OF BUISMUTH TRIOXIDE, TITANIUM DIOXIDE, BARIUM TITANTE AND LEAD OXIDE WHICH UNDERGOES A CHANGE IN COLOR UPON REACTION WHEN SUBJECTED TO A HIGH POTENTIAL NEGATIVE ION BOMBARDMENT CREATED BY A CORONA DISCGHARGE OF AT LEAST 10,000 VOLTS TO PRODUCE A DIFFERENTIAL CONDUCTIVE PATTERN COMPRISING RELATIVELY NON-CONDUCTIVE AND RELATIVELY CONDUCTIVE AREAS ON THE SURFACE THEREOF CORRESPONDING TO SAID GRAPHIC ORIGINAL, ESTABLISHING A HIGH POTENTIAL GRADIENT OF AT LEAST 10,000 VOLTS BETWEEN SAID COPYSHEET CONTAINING SAID DIFFERENTIAL CONDUCTIVE PATTERN AND A NEGATIVE CORONA ELECTRODE IN THE PRESENCE OF AN INOIZABLE ATMOSPHERE CONSISTING ESSENTIALLY OF NORMALLY GASEOIS MATERIAL, SAID ELECTRODE IN THE PRESENCE OF AN IONIZABLE FROM SAID COPY-SHEET THAT SUBSTANTIALLY THE UNTIRE CONDUCTIVE AREAS OF SAID COPY-SHEET ARE BOMBARDED WITH NEGTIVELY CHARGED NORMALLY GASEOUS IONS, AND CONTINUING SAID BOMBARDMENT OF SAID COPY-SHEET EITH SAID NEGATIVELY CHARGED NORMALLY GASEOUS IONS FOR SUFFICIENT LENGTH OF TIME TO PRODUCE A VISIBLE REPRODUCTION OF SAID GRAPHIC ORIGINAL UPON THE SURFACE OF SAID COPY-SHEET. 